Air may exist in a liquid in three different forms. One form is “free air,” which is the trapped air in a system but not totally in contact with the liquid; such as air pockets. Another form is “entrained air,” which exists in the form of bubbles in the body of the liquid, while the third form is “dissolved air” that is totally mixed with the liquid and exists at the molecular level. Free and entrained air usually get into the system through different means, such as violent agitation, a leak in a connection or seal, or the release of dissolved air due to a pressure drop (e.g., at pump inlets).
It is well known that the presence of air (or other gases) in a system, such as a hydraulic system, adversely impacts the performance of the system. First, air reduces the efficiency and consistency of a hydraulic liquid in transferring energy. Second, the movement of the air through one or more liquid channels in the system can cause unwanted noise. Third, air disrupts the expected heat transfer properties of the system. Other problems of aeration in a liquid channel include: changing the natural frequency of the system, the loss of lubricity, oxidation of system components, and excessive wear on system components (e.g., pumps). Premature failure of system components leads to increased service cost and greater operational downtime for machines.
For these reasons, methods and systems exist in the art to measure the aeration of a liquid. Aeration may be calculated according to the following equation:
      Aeration    ⁢                  ⁢    %    =                    Total        ⁢                                  ⁢        Air        ⁢                                  ⁢        Volume        ⁢                                  ⁢                  (                      entrained            +            dissolved                    )                            Liquid        ⁢                                  ⁢        Volume              ×    100    ⁢    %  The measured values in the above equation may be normalized to the standard temperature (20° C.) and atmospheric pressure if desired. Although the aeration may be defined and calculated based on both entrained and dissolved air, dissolved air may have negligible effect on liquid properties unless dissolved air is released and forms air bubbles (entrained air) in the liquid body.
There are several existing methods that are capable of quantifying entrained air in liquids. One method includes taking a sample of the liquid and then measuring the change in volume of the liquid as the air is allowed to escape from the sample. Other methods include the use of an infrared source focused on a liquid sample. U.S. Pat. No. 5,455,423 to Mount et. al. focuses an infrared source onto a venturi in a sample tube. The venturi is illuminated by the infrared source to detect and measure the amount of air bubbles in the liquid. Other methods to measure aeration known in the art include using X-rays to measure the density of the liquid. Still other methods examine the speed, temperature, and attitude of an engine relative to an axis (e.g., U.S. Pat. No. 6,758,187).
While these methods may detect the aeration of a liquid (specifically, the level of entrained air) to an adequate degree for some purposes, they have drawbacks. First, these methods require sampling the liquid from a working system to measure aeration. This often requires stopping the normal operation of the system or machine. Second, these methods may be costly. Third, the experimental setup of these methods may limit the ability to measure aeration levels at a specific location on a liquid system during normal operating conditions of a machine.
The present disclosure is directed to mitigating or eliminating one or more of the drawbacks discussed above.